Plane Stress Fracture Toughness Testing of Die Cast Magnesium Alloys 2002-01-0077

Plane stress fracture behavior was measured for magnesium alloys AM60B, AM50A, and AZ91D produced by high-pressure die casting. Compact Tension (CT) specimens were obtained from plate samples with approximately 2-5 mm thickness. The compliance unloading technique was used to record crack extension for each specimen. The AM50A and AM60B specimens exhibited stable crack extension beyond ASTM E 1820 limits for J_max (∼ 33 kJ m^-2 and 22 kJ m^-2, respectively) and Δa_max (2.1 mm and 1.3 mm, respectively). The data were in good agreement with a power law fit for J vs. Δa. The AZ91D samples had unstable crack extension, with a flat R-curve and a critical fracture energy J_c of ∼ 7.5 kJ m^-2. All fractures were by microvoid coalescence, initiated between the primary Mg grains and the brittle Mg₁₇Al₁₂ phase.